The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Semiconductor devices are generally manufactured on a silicon wafer. Each silicon wafer undergoes various processing steps to create multiple individual devices on the wafer. Each device on the wafer is referred to as a die. When the wafer is complete, the wafer is divided into the individual dies (or, dice). For example only, this dividing may be performed by sawing or by scoring and snapping.
The dice are generally vulnerable to their environment. For example only, the dice are physically fragile and prone to cracking and breaking when being handled. In addition, the dice are prone to contamination, both by physical contact as well as by airborne particles. The dice are also very sensitive to electrostatic discharge. For these and other reasons, the dice are often packaged. The packaged dice, or chips, are much easier to handle and may be easier to use in applications such as printed circuit boards.
A package may include one die or may include multiple dice. For ease of explanation, the following description will refer to a single die, although the present application applies to the packaging of multiple dice into a single package.
An exemplary package includes a first package piece to which the die is mounted. The first package piece includes a first face and a second face. The first face includes a die attach area where the die is placed. The second face may include features for electrically and mechanically connecting the package to a printed circuit board. Leads carry electrical signals from the die to the printed circuit board. Portions of the leads may be located on the first face of the first package piece.
When the die is secured to the die attach area of the first package piece, wire bonds may be formed between bond pads of the die and the leads. The leads may be bent so that they wrap around the edges of the first face and protrude past the second face. The leads can therefore be inserted into through-holes of a circuit board. Often, leads are attached to two opposing edges of the first face. This creates two rows of protruding leads, and the package is called a dual inline pin (DIP) package.
Through-holes may make circuit board layout more difficult and may prevent components from being placed on both sides of the circuit board. Surface mounting technology, where leads do not protrude through the circuit board, avoids these problems. The leads of a surface-mount package are generally bent to be parallel to the second face, and therefore present flat contacting surfaces to corresponding receiving pads on the circuit board.
Traditionally, leads have been located around the perimeter of the package. This is because the die occupies the center of the package and the bond wires connect outward from the die to the perimeter of the package. A surface-mounted version of the DIP package is a small-outline integrated circuit (SOIC), where the leads are bent outward when they reach the second face to form a gull-wing shape.
When the package has leads on all four edges, the package may be referred to as a quad flat package (QFP). When the leads do not extend beyond the edges of the package, the package may be referred to as a quad flat no-leads package (QFN). Some QFN packages include a central pad on the second face that dissipates thermal energy from the package to the circuit board.
To allow for greater connection density, leads may be formed in the interior area of the second face of the package. Internal wiring of the package may route signals from where bond wires connect at the perimeter of the package on the first surface to interior leads on the second surface of the package.
As described above, the face of the die has traditionally been exposed and bond wires connect the exposed bond pads of the die to leads of the package. In flip-chip packaging, the die is inverted onto the package, causing the bond pads of the die to contact the first face of the package. Then, the die can be designed to include bond pads in the center of the die, which will be located above the center of the package. This decreases the need for special conductors within the package that route signals from the perimeter of the first face to the center of the second face.
In one example, package leads are in the form of pins; such a package is called a pin grid array (PGA). Pin grid array packages may mount to a socket that is surface-mounted to the circuit board. In another example, the package includes an array of pads that connects to an array of pads on the circuit board. A ball of solder may be placed on each of the package pads; such a package is called a ball grid array (BGA).
A land grid array package includes pads that interface with pads on the circuit board. However, the land grid array package does not include balls of solder on the package pads. Instead, the circuit board may have a layer of solder on the pads. The solder may be restricted to the pad locations by using a mask when applying the solder. Once the package is positioned on the circuit board, the solder can be reflowed to secure each of the package pads to each of the receiving pads of the circuit board.
Alternatively, a socket may be connected to the circuit board. The socket applies clamping force to the package. This clamping force can either force the package pads to contact pads of the circuit board or cause the package pads to contact pads of the socket. A socket may allow for easy replacement of the package.
Once the die is mechanically and electrically connected to the first piece of the package, the die is covered. This isolates the die and any other components, such as bond wires, from the environment. The die may be covered by attaching a second package piece, such as a lid, to the first piece of the package. The first and second pieces may be sealed to prevent any contaminants from entering the space where the die is located.